This invention relates to the processing methods for the high-pure titanium, and more specifically, to the working methods for the high-pure titanium suitable for a titanium target for sputtering. In more detail, the invention relates to the methods of a cold plastic working for the titanium material (raw material) having a purity higher than 4N so as to obtain the titanium material having a fine grain size.
In manufacturing semiconductor devices, sputtering, vacuum deposition or ion plating is employed for forming a circuit material or a barrier metal in the form of a film on a semiconductor element. Of those methods, sputtering is generally used in practice. In sputtering, ions such as argon ions impact on a metallic target, thereby ejecting the metal ions, and which are piled on a base plate, resulting in forming a film. Many kinds of metallic targets are known, and among these, titanium targets are widely used for the semiconductor devices.
In order to produce a uniform thickness of the film on the semiconductor element, and to control the occurrence of so-called "particles" (which means a phenomenon that some large particles adhere on the film surface in spattering), the grain size of titanium targets must be about 20 .mu.m or smaller. After preparing the titanium material by forging and rolling, the grain size is controlled by recrystallization and annealing to satisfy the requirements for the titanium target mentioned above. For example, Japanese Patent Unexamined Publication (Kokai) No. 8-232061 discloses a method that the matrix of a titanium ingot was broken by drawing and upsetting in the temperature higher than the transformation temperature (882.degree. C.), and performing the same forging as mentioned above in the temperature lower than that of the transformation. The disclosed method allows the matrix to accumulate working strain so as to reduce the grain size in the matrix. In addition, Japanese Patent Unexamined Publication (Kokai) No. 8-269698 and No. 8-333676 discloses a method that the grain size in the titanium targets are reduced by rolling or forging in the temperature lower than that of the transformation.
The conventional methods disclosed in the publications for reducing the grain size require at least a heating equipment in forging and/or rolling at a temperature from 400 to 800.degree. C., leading to the high operation cost regarding such as an electric consumption, thereby having disadvantages in view of the cost. In addition, the methods mentioned above accompany the scale growth on the surface of the titanium material, and the additional descaling process, which complicates the subsequent process.
An object of the invention is to provide a method for processing the high-pure titanium having an average grain size of 50 .mu.m or smaller, preferably of 40 .mu.m or smaller, and more preferably of 35 .mu.m or smaller, in which the scale growth is prevented and at comparatively low cost.